There are numerous methods other than vertical or angled roof bolting for supporting the roof of an underground mine which is dug for the extraction of coal or other minerals or ores. One such method is the crossbar method wherein wood, steel, rail, fiberglass, or aluminum beams are placed against a mine roof. Each end of the beam can be supported by posts made from any of the materials used in making the beams or, in addition, from concrete. The crossbar method has the disadvantage that the posts can be accidentally knocked out by moving machinery, thus endangering the miners. To protect miners in such situations, cable or steel straps are bolted into the roof in order to support the beam should a post be knocked out. The beams can also be drilled and bolted directly to the roof. Installing crossbars is a slow and labor-intensive process, the materials are expensive, and installation can be hazardous. Moreover, wood is not a permanent material even if it is treated.
In another method, continuous bolt trusses are fabricated from angled roof bolts anchored into the roof by mechanical shells or adhesive resins. The bolts are connected by means of a tie-rod and tension is produced by a turnbuckle. Tie-rods or other length compensating devices are connected in sections of from two to five connecting rods. The tension in the bolts and tie-rods is essentially equal as they are continuously connected together. Tightening of the tie-rod turnbuckle can produce compressive forces in the roof rock which increases the strength of the rock. However, as the turnbuckle length or take-up is limited, the roof bolt holes must be precisely located or, otherwise, various lengths of tie-rods must be available to be connected to the roof bolts and turnbuckles in order to allow the truss system to be tensioned. The threads, which are cut or rolled into the ends of the roof bolts and tie-rods, act as stress concentration points and also reduce the effective area of the bolt/tie-rod, thus reducing the effective ultimate strength of the system. Fine machine threads are subject to damage, rust, and corrosion. Assembly of the continuous bolt multi-segment tie-rod truss system is time-consuming.
In a third method, multiple angled bolt trusses are fabricated by securing one end of each of two bolts at angles in the roof of the mine and by passing the other ends of the two bolts through plates or brackets such that each bolt is tensioned separately. Tie-rods, in two to five sections, are connected to the plates or brackets using turnbuckles or tensioning bolts and couplers such that the turnbuckles or tensioning bolts can tension the tie-rods. Since the tie-rods and bolts are tensioned separately, the compressive forces on the roof rock may be unequal. This may result in one bolt being overloaded close to failure while the tie-rod and opposite bolt have little or no stress. The roof bolt holes must be located at precise distances to allow tensioning within the limited range of a turnbuckle or tensioning bolt or else several sections of various lengths of tie-rods must be available to achieve the proper tie-rod length. The threads which are cut or rolled into the threaded ends of the roof bolts and tie-rods act as stress concentration points, and also reduce the effective area of the bolt/tie-rod thus reducing the effective ultimate strength of the truss system. Fine machine threads are subject to damage, rust, and corrosion. Assembly of the roof truss system is time-consuming.
In a fourth method, cable bolts or slings of lengths of wire rope are inserted into bore holes in the roof of a mine on either side of the mine passageway. Grout or grout cartridges are inserted into the holes in order to secure the cables to the mine roof. As each cable is one continuous piece, the tension in each cable can be equal in all segments. The cable or sling can be tensioned by attaching it to a split tube and by driving the split tube up into the cement-filled bore hole by a split tube driver which can be an air or hydraulic impact tool. However, variations in bore hole diameter due to drilling and/or rock movement hinder the passage of the split tube such that there is little control of the tension on the cable or sling. After installation, some cables have no tension and must be blocked with wood to the roof and tightened with wedges. Also, the tubes require special air or hydraulic jack legs which are not normal coal mining equipment, thus requiring an air compressor and air hoses laid to the work area. Moreover, the impact driving of the split tubes is slow and very noisy, and requires three operators to install a cable sling. Furthermore, impact driving of the split tubes can disturb the roof and ribs and may dislodge material thus endangering miners.